Chapter 14 - Digestive Systems

How does the digestive system contribute to homeostasis?

by transferring nutrients, water, & electrolytes from the external environment to the internal environment

What are the main fuel types and their absorbable units?

carbohydrates → simple sugars

fats → fatty acids

proteins → amino acids

Complete Digestive System

digestive complex consisting of a long, continuous tube that extends from an entrance (mouth) to an exit (anus)

• one-way digestive tract

• allows simultaneous operation of sequential stages in the processing of food

• reduces mixing of digested and undigested matter

Lumen

the open space within the digestive system; part of the external environment

Crop

muscular sac-like outpouching of the esophagus in some bird species; temporary storage area for food

• crop milk = high in lipids; secreted into the crop of some birds, regurgitated, & fed to young

Filter/Suspension Feeders

primarily feeds by trapping organic material suspended in water

Detritivores (Deposit Feeders)

primarily feed on dead & living material in sediments

Fluid Feeders

primarily feed by sucking or licking fluids from larger organisms

Carnivores

primarily feed by eating other animals

Herbivores

primarily feed by consuming algal or plant material

Omnivores

feed on both animals & plants

Symbiotic Autotroph-Bearing Animals

animals that obtain food from photosynthesizers or chemosynthesizers

Four Basic Digestive Processes

1. motility

2. secretion

3. digestion

4. absorption

Motility

muscular contractions mix the contents of the digestive tract & move them forward

• smooth muscle in the walls of the digestive organs is regulated by complex autonomic mechanisms

• skeletal muscle at the two ends of the digestive tract is under voluntary control

Autonomous Smooth Muscle Function

• slow-wave potentials = fluctuations in membrane potential

• sheets of smooth muscle cells are connected by gap junctions, forming a functional syncytium

• if threshold is reached, a volley of action potentials is triggered

  • the opening of v-g Ca²⁺ channels results in Ca²⁺ influx into smooth muscle cells

• intensity/strength of contractions depends on the number of action potentials produced

Slow-Wave Potentials

rhythmic, wave-like changes in the resting membrane potential of gastrointestinal smooth muscle, acting as a pacemaker for gut motility

• Basic Electrical Rhythm (BER) - fundamental rhythm of spontaneous electrical activity in the smooth muscle of the digestive tract

Smooth Muscle Contraction Mechanism

Summary of Digestive Control

Enteric Nervous System

complex network of neurons within the gut wall that controls and regulates the digestive system

• referred to as the "second brain" because of its extensive neural circuitry and ability to function independently of the CNS

Secretion

digestive juices are secreted into the lumen of the digestive tract by exocrine glands

• each secretion contains water, electrolytes, & organic molecules important in the digestive process

Digestion

complex food molecules are broken down into smaller absorbable units by enzymatic hydrolysis

Absorption

small absorbable units, along w/ water, vitamins, & electrolytes, are transferred from the digestive tract into blood

• involves specialized transporters in epithelial cells

• surface area is greatly increased to enhance absorption

  • some aquatic species can absorb small nutrient molecules through the gills or epidermis

Components of the Digestive “Tube”

mouth, pharynx, esophagus, stomach, small intestine, large intestine, & anus

• ruminants have a rumen, a first stomach section meant for storage

• birds have the proventriculus-gizzard complex (bird stomach)

What are the accessory organs to the digestive system?

salivary glands, exocrine pancreas, & the biliary system (liver & gallbladder)

Mouth

primary function: obtaining & receiving food

• Palate: separates the mouth from the nasal passages

  • hard palate = bone

  • soft palate

• Tongue: captures & guides food within the mouth; also used for taste, vocalization, & synthesis of antibiotic peptide

  • voluntarily controlled skeletal muscle

Specialized Mouth Shapes

• snakes have hinged jaws & powerful muscles in the cheek & throat for swallowing prey whole

• beaks of birds have been molded to conform to particular feeding habits

• muscular lips in mammals aid in seizing & conveying food to the mouth (prehension)

Mastication

“chewing”; grinds & breaks food up into smaller pieces to facilitate swallowing & increase surface area for mixing w/ saliva

• mixes food w/ saliva

• stimulates taste buds

Teeth

hard, calcified structures embedded in the jaws that are used for chewing and breaking down food; exposed part is covered by enamel (the hardest structure in the body)

• teeth of rabbits & lower incisors of rodents grow continually to replace lost enamel coating

• sharks & reptiles replace teeth throughout life (polyphyodontia)

• baleen replaces teeth in baleen whales for filter feeding

Saliva

liquid containing water, mucus, & salivary amylase; produced by salivary glands

• parotid, sublingual, & submandibular glands in mammals

• continuously secreted in mammals

  • ruminants produce copious amounts of alkaline saliva (ex. cattle produce 140 L/day)

Saliva Functions

• moistening food (lubrication provided by mucus)

• digestion via salivary amylase & lingual lipase (most digestion occurs further down the digestive tract)

• defense against ingested bacteria

• taste (only molecules in solution reach taste buds)

• neutralization of acid

• thermoregulation (evaporative cooling in panting

• poisons (ex. reptile venom)

• anticoagulation in blood sucking animals

• pheromones

Simple (Unconditioned) Salivary Reflex

occurs when chemoreceptors & pressure receptors in the oral cavity respond to the presence of food

• salivary center in medulla oblongata stimulates autonomic neurons

Acquired (Conditioned) Salivary Reflex

occurs w/o oral stimulation; “mouth-watering” anticipation

• learned response based on previous experience

Salivary Reflex

can be simple (unconditioned) or acquired (conditioned) & is under autonomic control

• both parasympathetic & sympathetic neurons stimulate saliva production, but parasympathetic dominates

• sympathetic stimulation produces more mucus = dry mouth sensation during stress

Pharynx

cavity @ the rear of the oral cavity; common passageway for both the digestive & respiratory systems in vertebrates

Esophagus

fairly straight muscular tube extending between the pharynx & stomach; skeletal muscle gives way to smooth muscle in most vertebrates

Swallowing

the process of moving food from the mouth through the esophagus into the stomach

• bolus of food is forced by the tongue into the pharynx

• pharyngeal pressure receptors send afferent impulses to the medulla oblongata

• swallowing center activates programmed all-or-nothing sequence of highly coordinated activities

• swallowing reflex has 2 phases: oropharyngeal phase & esophageal phase

Oropharyngeal Phase of Swallowing

involves a series of voluntary and involuntary movements that protect the airway and propel the food bolus downwards

Esophageal Phase of Swallowing

ring-like peristaltic contraction sweeping down the esophagus

Stomach Functions

storage of food, digestion of proteins (initiation), & formation of chyme

• Chyme: thick, liquid mixture of food & gastric secretions

Steps of Gastric Motility

1. filling

2. storage

3. mixing

4. emptying

Gastric Filling

deep folds in the stomach wall flatten as the stomach relaxes w/ ingestion (receptive relaxation)

Gastric Storage

weak contractions in fundus (top of stomach) & body allow storage of food in the body & a pocket of gas in the fundus

Gastric Mixing

strong peristaltic contractions mix food w/ gastric secretions in the antrum (bottom of stomach); w/ pyloric sphincter closed, chyme tumbles back for more mixing

Gastric Emptying

small amounts of chyme pass into the duodenum w/ each contraction before the pyloric sphincter closes

• stomach empties @ a rate proportional to the volume of chyme in it

  • higher volume → more stretch → stretch-activated channels in smooth muscle excite the cells → activation of v-g calcium channels & contraction

• duodenal factors affecting gastric emptying are fat content, acidity, hypertonicity, & distension

How does the volume of stomach content affect the rate of gastric emptying?

higher volume → more stretch → stretch-activated channels in smooth muscle excite the cells → activation of v-g calcium channels & contraction

Factors Regulating Gastric Motility & Emptying

Gastric Secretion

cells responsible for gastric secretion are located on the inner surface of gastric pits in the stomach lining (gastric mucosa)

→ oxyntic mucosa in the body & fundus

• mucous cells secrete a thin, watery mucus

• chief cells secrete pepsinogen

  • pepsinogen is inactive until cleaved by HCl or pepsin to form pepsin

  • pepsin digests proteins in a low pH environment

• parietal cells (oxyntic cells) secrete HCl & intrinsic factor, which delivers vitamin B12 to ileum

→ pyloric gland area in antrum

• endocrine G cells secrete gastrin into the blood

HCl Secretion Mechanism

• splitting of H₂O into OH^- & H⁺

• H⁺/K⁺ATP-ase drives H⁺ secretion into lumen

  • passive K⁺ transport back out of the cell

• secondary active transport of HCO₃^- out & Cl^- into the cell (Cl^- passively transports out)

  • HCO₃^- created by combination of CO₂ w/ OH^-

Pepsinogen Activation in Lumen

Control of Gastric Secretion

• Cephalic Phase: anticipatory secretion of HCl & pepsinogen due to food-related stimuli

  • stimulated by acetylcholine & gastrin

• Gastric Phase: stimuli acting in the stomach— protein (most potent stimulus), distension, caffeine, & alcohol

• Intestinal Phase

  • excitatory component = presence of protein fragments in duodenum

  • inhibitory component = same factors that inhibit gastric motility also inhibit gastric secretion

Digestion in the Stomach

• carbohydrate digestion continues due to swallowed salivary amylase

  • amylase is destroyed by stomach acid, but the interior of swallowed food masses is protected

• protein digestion begins in the antrum w/ the action of HCl & pepsin

  • proteins are hydrolyzed into smaller peptides

• stomach lining is protected from digestion by the gastric mucosal barrier

  • secretion of bicarbonate-rich mucus

  • stomach lining is replaced every 3 days

Pancreas Secretion

exocrine pancreas secretes digestive enzymes & an aqueous alkaline fluid

• acinar cells secrete proteolytic enzymes, amylase, chitinase, & lipase

  • pancreatic amylase converts polysaccharides → disaccharides

  • pancreatic chitinase (fish/some marine birds) breaks down chitin

  • pancreatic lipase hydrolyzes triglycerides → monoglycerides & 3 fatty acids

  • sodium bicarbonate secreted by pancreatic duct cells

• proteolytic enzymes are secreted in inactive form

  • activated in the duodenum by enterokinase and/or trypsin

  • active forms = trypsin, chymotrypsin, & carboxypeptidase

  • each attacks a different peptide linkage

Exocrine vs. Endocrine Portions of the Pancreas

What is the importance of sodium bicarbonate secretion in the digestive system?

to neutralize acidic contents arriving from the stomach

Hormonal Control of Pancreatic Exocrine Secretion

Hepatocytes

liver cells; perform a wide variety of metabolic & secretory tasks

Liver Functions

• secretion of bile salts & hormones

• metabolic processing of nutrients

• detoxification of body wastes, hormones, & drugs

• synthesis of plasma proteins, lipoproteins, & ascorbic acid (vitamin C)

• storage of glycogen, fats, iron, copper, & vitamins

• activation of vitamin D

• removal of bacteria & worn-out RBCs

• excretion of cholesterol, biliverdin, & bilirubin

• buoyancy in sharks

Hepatocytes secrete bile into the…

bile canaliculi (feeds into the bile duct)

Hepatic Portal

unique circulatory pathway that directs blood from the digestive organs to the liver before it enters the general circulation

• ensures that the liver can process nutrients absorbed from the gut and filter out any toxins before they reach the rest of the body

Biliary System

a network of organs and ducts involved in the production, storage, and transport of bile

• the liver secretes bile & the gallbladder stores it

• sphincter of Oddi prevents bile from entering the duodenum except during digestion

  • diverts bile secreted by the liver into the gallbladder between meals

• bile salts are recycled through the enterohepatic circulation

Structure & Function of Bile Salts

bile salts aid in the digestion of fats by forming micelles around lipid molecules

• micelles disperse through the watery contents of the lumen

• break down fats into smaller, digestible particles

• facilitate the absorption of fat-soluble vitamins and cholesterol

Biliverdin

a green bile pigment produced during the breakdown of heme (from worn-out RBCs)

• precursor to bilirubin (in mammals)

Bilirubin

a yellow pigment produced during the natural breakdown of red blood cells excreted into bile

• if bilirubin is formed too quickly, it accumulates, causing jaundice

Small Intestine Division

the small intestine is divided into the duodenum, jejunum, & ileum

4 Layers of Digestive Tract Wall

from inside to outside:

1. mucosa

2. submucosa

3. muscularis externa

4. serosa

Small Intestine Motility (1)

segmentation both mixes & slowly propels chyme

• oscillating ring-like contractions of circular smooth muscle

• initiated by basic electrical rhythm (BER) periodically reaching threshold

• frequently declines along the length of the small intestine, facilitating forward progress

Factors Influencing Intensity of Small Intestine Segmentation

factors that influence the excitability of smooth muscle cells include:

• distension of the intestine (especially important in duodenum)

• gastrin (especially important in ileum)

  • gastroileal reflex = peristalsis of the ileum & opening of ileocecal valve

• extrinsic nerve activity

  • strength of contraction enhanced by parasympathetic activity

  • strength of contraction diminished by sympathetic activity

Small Intestine Motility (2)

• ileocecal valve & ileocecal sphincter prevent contamination of small intestine by colonic bacteria

• gastrin secreted in response to food entering the stomach causes ileocecal valve to relax

Small Intestine Structure

the small intestine has a large surface area for absorption; the following features increase surface area 600-fold

• inner surface is folded

• villus = microscopic finger-like projections; each villi contains a capillary network & terminal lymphatic vessel (central lacteal)

• microvilli = smaller hair-like projections arising from the luminal surface of epithelial cells (forming brush border)

By how much does a fold of the small intestine increase absorptive surface area?

3-fold

By how much does a villus of the small intestine increase absorptive surface area?

10-fold

By how much does microvilli of the small intestine increase absorptive surface area?

20-fold

Where does most absorption take place in the small intestine?

duodenum & jejunum

Absorption of Carbohydrates

disaccharides are hydrolyzed into absorbable monosaccharides by enzymes in the brush border

• glucose & galactose are absorbed via secondary active transport w/ Na⁺ as a cotransported species

• facilitated by Na⁺/K⁺ATPase

Carbohydrate Digestion

Absorption of Proteins

amino acids are absorbed by secondary active transport similar to glucose transport

• peptides & amino acids cotransported w/ Na⁺ & H⁺

• facilitated by Na⁺/K⁺ATPase & Na⁺/H⁺ATPase

Protein Digestion

Absorption of Fat

once micelles reach the epithelial cells, monoglycerides & free fatty acids diffuse across the luminal membrane

• reassembled into triglycerides inside epithelial cells

• chylomicrons, containing triglycerides, are extruded by exocytosis into the interstitial fluid & picked up by lacteals

Absorption of Vitamins

• water-soluble vitamins are absorbed passively

• fat-soluble vitamins are absorbed along w/ fat

• vitamin B₁₂ must be combined w/ gastric intrinsic factor (secreted by stomach) for absorption in the ileum

Absorption Volume

7 liters of secreted digestive juices must be absorbed daily

• diarrhea is a passage of a highly fluid fecal matter, usually due to excessive intestinal motility

• loss of fluids = dehydration, loss of nutrients, & metabolic acidosis

Countercurrent Exchange & Water Absorption

Large Intestine Division

consists of the cecum, colon, & rectum/cloaca

• vermiform appendix in humans & some apes stores lymphocytes & has no digestive function

• 3 regions of the colon: ascending, transverse, & descending

Large Intestine Differences

• carnivores = short, simple colons

• herbivores & omnivores have longer colons

• in amphibians, reptiles, birds, & some mammals, the large intestine terminates in a cloaca, shared by the renal & reproductive systems

Hindgut

another word for large intestine

Colon Function

the primary function of the colon is storage of fecal matter

• symbiotic microorganisms synthesize vitamins (ex. vitamin K)

• undigested cellulose adds to the bulk

• ex. human colon

  • receives 500 mL of indigestible chyme from the small intestine every day

  • absorbs water & salt

Large Intestine Motility

peristaltic contractions propel the contents toward the rectum, while antiperistaltic contractions fill the cecum

• bacteria accumulate in the large intestine bc of the slow colonic movement

• after meals, mass movements quickly drive colonic contents into the distal portion of the large intestine where material is stored until defecation

Defecation

1. distension of the rectum stimulates stretch receptors in the rectal wall initiating a defecation reflex

2. internal anal sphincter (smooth muscle) relaxes & the rectum & sigmoid colon contract more vigorously

3. defecation occurs when the external anal sphincter (skeletal muscle) is voluntarily relaxed

What does fecal matter consist of?

undigested food (ex. cellulose), bilirubin or biliverdin, small amounts of salt & water, & bacteria

Gastrin

gastrointestinal hormone that stimulates secretion of gastric juices & enhances motility in several areas

Secretin

gastrointestinal hormone that acts by several mechanisms to reduce acidity in the duodenum

Cholecystokinin (CCK)

gastrointestinal hormone that inhibits gastric motility & secretion, stimulates secretion of pancreatic enzymes & bile release, & signal satiety

Motilin

gastrointestinal hormone that stimulates motility in the stomach & small intestine

Ghrelin

gastrointestinal hormone that stimulates growth hormone release & increases appetite

Leptin

gastrointestinal hormone that suppresses appetite